Lough Allen (Irish: Loch Aillionn) is a lake on the River Shannon in northeastern Connacht, Ireland. Most of the lake is in County Leitrim, with a smaller part in County Roscommon. The lake lies to the south of the River Shannon's source, near the Iron Mountains, and is the uppermost of the three main lakes on the river. The other two, Lough Ree and Lough Derg are much further to the south.
The lake is shaped like an upturned isosceles triangle. The Shannon enters the lake at the wider northern end and leaves the lake at the narrow southern end. Other rivers that feed the lake include the Diffagher (northwest), the Yellow (northeast), the Stoney (east) and the Arigna (southwest). The R280 regional road skirts the west side of the lake, while the R207 follows the east bank, from Ballinagleragh to Drumshanbo. The R200 road is on the north side of the lake, traveling west from Dowra to Drumkeeran. The Iron Mountains (Slíabh an Íariann) lie to the east and west of Lough Allen.
Some claims have been recently made that volumes of oil and gas lie beneath the lake and the Allen basin.